Environmental control process for gaseously removing carbon from liquid metals



United States Patent 3,506,436 ENVIRONMENTAL CONTROL PROCESS FOR GASEOUSLY REMOVING CARBON FROM LIQUID METALS Norman A. D. Parlee, Los Altos Hills, and William E. Mahiu, Oakland, Calif., assignors to Kaiser Industries Corporation, Oakland, Calif., a corporation of Nevada No Drawing. Filed Nov. 25, 1966, Ser. No. 596,782 Int. Cl. C21c 7/00, 7/04, 39/14 US. CI. 75-59 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for removing carbon from a liquid metal containing an easily oxidizable element other than carbon without seriously oxidizing that other element. This is accomplished by maintaining the oxygen reactivity in the melt at such a level that it will have a definite oxidizing potential for the carbon without significantly oxidizing the other element. This is accomplished as shall be explained herein by utilizing the relationship between the following two reactions:

This invention relates to removing undesired constituents from metals. More particularly, it is directed to a process for removing carbon from a liquid metal containing an easily oxidizable element other than carbon without seriously oxidizing that other element. The instant process has particular application to removal of carbon from steels, such as stainless or silicon steel, without oxidizing other easily oxidizable constituents of the steel.

A number of applications for metals require very low carbon contents in the metal. For example, the high chromium, nickel stainless compositions frequently are desired at carbon contents of 0.03% by weight maximum in order to provide superior resistance to corrosion. Similarly iron and silicon iron sheets for all types of electromagnetic devices customarily are used at carbon contents of 0.01% by weight maximum in order to provide superior magnetic properties. However, to obtain such low carbon contents in these steels is difficult because carbon usually is present in the raw materials in excess of the desired amounts.

The lowering of the carbon content of metals has been the subject of extensive research in the metallurgical field. It is difficult to selectively reduce the carbon content when the metal contains other easily oxidizable constituents in addition to carbon. Many solutions have been proposed to overcome this problem, however, none have been completely satisfactory. In the case of stainless steel, for example, where one of the other easily oxidizable constituents is chromium, it has sometimes been the practice to start with a large excess of chromium so that some may be oxidized during the decarburization of the metal leaving behind the desired final quantity of chromium. This obviously is wasteful of chromium and leads to undesirable high chromium slags. Another prior art method has been to let the chromium oxidize and then subject the decarburization metal and slag to a subsequent chromium reducing step. This, of course, involves an extra step in the process with resultant complications and higher costs. Thus, no completely satisfactory method of decarburizing stainless steel has been developed prior to the instant invention.

It is an advantage of the instant invention that it affords means of removing carbon from metal, such as steels con- Patented Apr. 14, 1970 taining an easily oxidizable element other than carbon such as chromium or silicon, without oxidizing the other easily oxidizable elements. As applied to stainless steel, the instant process advantageously produces essentially slag-free stainless metal with very low carbon content. Because of this, an additional advantage of the instant process is that it reduces scarfing costs caused by slag and deoxidation product inclusions in the finished metal, since the occurrence of these inclusions is greatly reduced if not eliminated. Because of the simplicity of the process and of the high quality product it produces, an additional advantage is that it makes possible appreciable savings in the overall cost per ton of metal of the desired quality produced.

The instant invention takes advantage of the following reactions:

(1 m re) I 2(g)+ Q (2 C Q C (a) As can be seen from the above, Equation 1 can be used to control the dissolved oxygen (0) concentration in the metal by controlling the ratio of H O/H Thus, the oxygen level can be maintained below that level which will oxidize to a significant extent dissolved elements other than carbon. Since reaction (2) between dissolved carbon (C) and dissolved oxygen (0) involves the formation of CO gas, controlling the concentration of CO in the environment will control the extent of this reaction. This can only be accomplished, however, as should be apparent, if the concentrations of the various constituents are rather carefully maintained.

As used herein the terms environmental control processes refers to a family of processes for improved metal making in a controlled innocuous atmosphere environment.

Accordingly the instant invention involves a process for removing carbon from a metal containing an easily oxidizable element, other than carbon, which comprises first providing a body of molten metal in a chamber maintained above the melting point of the metal. Innocuous gas is passed into the chamber in a sweeping manner over the metal so as to maintain an atmosphere therein substantially free of undesired constituents.

In this specification and the appended claims, the term innocuous gas is defined as a gas or mixture of gases that does not react appreciably to form reaction products that are undesirable in the metal at conditions of use. Some examples of innocuous gases are the noble gases and for steel in the instant process hydrogen, water vapor and in some instances certain concentration ranges of carbon monoxide and carbon dioxide, and even some mixtures of steam and partly burned natural gas. A hydrogen vapor content is maintained in the innocuous atmosphere above 2% by volume and the water vapor content in the innocuous atmosphere is maintained at a volume ratio to the hydrogen so as to maintain an oxygen concentration in the molten metal which reacts with carbon in the molten metal to form carbon monoxide gas without substantially oxidizing the other oxidizable elements. The carbon monoxide gas is stripped or gettered from the innocuous atmosphere as necessary so as to cause the desired reactions to occur. In maintaining the hydrogen vapor content and the water vapor content in the innocuous atmosphere at the desired level, it sometimes may be necessary to add hydrogen and/or water or sometimes it may be necessary to getter or strip hydrogen and/or water from the innocuous atmosphere. When the carbon content of the molten metal is reduced to the desired level, the hydrogen and water vapor contents of the innocuous atmosphere are lowered so as to remove hydrogen gas from the molten metal until the hydrogen content of the metal is at the desired level. In this way, a metal containing easily oxidizable elements other than carbon substantially free of undesired carbon impurities is 4 From the above table, it can be seen that all stainless steels contain substantially more than 10% chromium and most contain more than 15% chromium. In the making of stainless steel, the chromium-oxygen relationship is very important. Although chromium is only a relaproduced. tively mild deoxidizer, 11s presence in such large concen- Among the easily oxidizable elements which can be tration makes it a stronger deoxidizer than the carbon protected from oxidation in this manner are elements and manganese and often stronger than the silicon (deselected from the group consisting of iron, chromium, pendent on amount) present. This is largely because, alsilicon, manganese, vanadium, nickel and the like. As has though there may be still ccnsiderable oxygen present, been stated, the instant process is particularly applicable the chromium lowers the oxygen reactivity or activity to to the removal of carbon from ferrous metals. When the below the level where it can form oxides with the carferrous metal contains chromium, the process is most bon (approximately 0.10%), manganese (approximateeffective when the water vapor content in the innocuous ly 1.5%), and often the silicon (approximtaely 0.8%). atmosphere is maintained at a volume ratio to the hydro- 15 h s, even in the absence of strong deoxidizers such as gen of from about 0.01 to about 0.06 so as to maintain aluminum, the chromium is powerful enough to prevent an oxygen concentration in the molten metal of not more the formation of carbon monoxide gas and the formation th 0,12% by weight, Thi i tfi i t t ea t i h the of silicon or manganese inclusions While the liquid metal carbon present in the metal without appreciably or sigis Stabililed for fixample 1500 0 Well above its nificantly reacting With the chromium. Although the freezing p It also Pfsvshts carbon mQhOXide 8 hydrogen vapor content in the innocuous atmosphere can mation on g- This means also that, at 1600 it be maintained at any desired level above 2% by volume, any Considerable amount of oxygen is present in the atso long as the Water vapor ratio criterion is met, it is parmosphere above the metal, the Chromium Will react With ticularly efiicacious to use an innocuous atmosphere that it faster than Will carbon, manganese, normal silicon, is substantially 100% hydrogen. In this case, the water and form a high chromium ide layer on the Surface of vapor content in the innocuous atmosphere is maintained the melt which can lead to hsaVY Chromium lossat a volume ratio to the hydrogen of from about 0.01 to Possibly the best y to explain this is to look at the 0.06 so as to maintain an oxygen concentration in the equilibrium between dissolve-d chromium, dissolved Y- molten metal of not more than 0.12% by weight. For gen and solid a -1 in illiquid 13% chromium iron most commercial metals, satisfactory results are realized 3O 3 at when the carbon content of the molten metal is reduced TABLE H to the desired level to lower the hydrogen and water vapor Degrees 1500 contents of the ll'll'lOCllOllS atmosphere to remove hydropercent 0 004 t t i ufl m tlltle molterli metatl unt1l ttllile hi srog coli- Activity on the percent scale of oxygen 0.006 en 0 e mo en me a is no more an arts er million. If desired, additional oxygen can be ren ioved by Thus, although t l600 C. there may be 0.04% d1sutilizing the concepts disclosed in our copending applica- Solved oxygen by Weight present In an chrommm tion Ser. No. 596,898, filed Nov. 25, 1966. In other words, that i of xygen acts A were a hydrogen content in the final metal product of not more winch much less than requlmd to bnng man about 6.5 pans per million is generally satisfactonh 40 about reactions w1th the carbon and manganese as shown If desired, of course, the hydrogen content can be further below In Table low r d, TABLE IIL-APPROXIMATE CONCENTRATION OF OXY- When the metal treating chamber temperature is above 9 E 1 ir d l t iim fiii h R E sE iv l g izs o i H THE C about 1750" C. it is particularly advantageous to maintain the water vapor-hydrogen ratio at not more than about n oi iiiti li 0.06 so as to maintain an oxygen concentration in the Present P Oxygen requiredm molten metal of not more than about 0.12% by weight. Element percent activity react percent This is because the reactivities of the various o tit t 0 0-035 0-23stS1stm0sC0 with oxygen arehsomlelwhat telmperature sensitive. For the $1 (loWerC grade). 0. 02 8. 23 gg i niii iiinai). Same reason, W en t e meta treating chamber tam 1 n ture is about 1600 c., it is desirable to maintain a finer a p y i ratio at not than about so as Thus, it is only the 0.8% silicon that approaches the t0 ma1ntam an oxygen concentration in the molten metal deoxidizing power of chromium and is roughly equivalent of not more than about to it (both at oxygen activities of approximately 0.006). As has been stated abOVe, the Instant itlvsntiofl is P This demonstrates that if the silicon is low, then the ticularly suited for the removal of carbon from stainless hromium i th stronger deoxidizer, If the ili on exsteels. Table I below gives the compositions of several ceeds some figure, perhaps about 0.8%, the silicon is stainless steels. the stronger deoxidizer and, thus, can reduce Cr 0 from TABLE I.SIAINLESB STEEL GRADES Nominal composition, weight percent Mn, s1, A.I.S.l., type C, Max. Max. Max. Cr Ni Other Max.

0. 0s 2. 00 1. 00 18-20 8-12 0. 03 2. 00 1. 00 13-20 8-12 0.15 2. 0o 1. 00 16-18 6-8 0.08 2.00 1. 00 16-18 10-14 3M0 0.03 2.00 1.00 16-18 10-14 3M0 0. 15 2. 00 1. 00 17-19 18-10 0.12 1. 00 1. 00 14-18 0.15 1.00 1.00 11. 5-13. 0.15 7.5 1.00 1s-1s 3.5-5.5 0.25N

The following discussion of the reaction between carbon and oxygen is made on the basis that the carbon monoxide formed is at a partial pressure of one atmosphere unless otherwise stated.

the slag. This explains why about 1% silicon has been tried in the past to hold chromium losses to a minmium in commercial practices not utilizing environmental control principles as does the instant process.

The carbon-oxygen relationship in all steels, including stainless, is controlled mainly by the reactions of dissolved carbon (C) with dissolved oxygen (0) to form carbon monoxide gas.

a a =0.002, for p =l atmos or (f =percent C) (f xpercent 0)=0.002

reasons it frequently will be desirable that the protection should be twice as good or a volume ratio of water vapor to hydrogen of about 0.011 to 0.016. As the reaction of the carbon with the oxygen proceeds, of course, the carbon monoxide that forms must be stripped or gettered from the innocuous atmosphere in order to maintain the driving force for the reaction. At any given level of dissolved oxygen in the metal, the extent to which the carbon removal by oxygen to form carbon monoxide will proceed depends upon the partial pressure of the carbon monoxide gas that is maintained in the atmosphere that is in contact with the metal. The lower the partial pressure of the carbon monoxide, the greater will be the extent of the removal of the carbon.

Table IV below gives the carbon, oxygen relationships in type 304 stainless steel at 1600 C. in environmentally controlled atmospheres according to the practice of the instant invention.

TABLE IV.CARBON, OXYGEN RELATIONSHIPS IN TYPE 804 STAINLESS AT 1,600 0., ENVIRONMENTAL CONTROL ATMOSPHERE 1.0 percent 00 1.0 percent 00 10.0 percent 00 1.0 percent 00 4.0 percent H: 32.0 percent H; 90.0 percent Hg 1 99.0 percent Hg 1 2 H20, H20, H20, p.p.m. 0, percent p.p.m. 0, percent p.p.m. 0, percent p.p.m. 0, percent 1 Dry basis. These activity coefficients (these do not change drastically with temperature) have been calculated for type 300 steels for 1600 C. as follows:

Thus the C-0 relation for type 304 at 1600" C. becomes (0.58 percent C) (0.15 percent 0)=0.002

Percent C O=0.023, at p =1 atmos or Percent CXa =0.0034, at p xl atmos It was this relationship that was used to show that the oxygen activity in type 300 (largely due to 18% chromium) is far too low (0.006 or 0.04% in simple theory but more like 0.003 or 0.02% in actual practice) to bring about any carbon boil, that is formation of CO at a partial pressure of one atmosphere or more, which would require an oxygen activity of at least 0.035 (approximately 0.23%) in a 0.10% carbon steel, and even more in a 0.15% carbon steel. Because of these considerations, carbon boils without great loss of chromium are only brought about at extremely high temperatures in conventional practice. This is because these and other equilibrium constants are altered at these very high temperatures where oxidation of carbon can be favored somewhat over chromium. It is an additional advantage of the process according to the instant invention that subjecting the metal to these very high temperatures can be avoided.

For these and other reasons atmospheres are used in the instant process that prevent oxidation of these easily oxidizable elements and at the same time have a definite oxidizing potential in the molten metal. If the chromium in, for example, type 300 stainless is not to be oxidized, the activity of oxygen in the melt and in the atmosphere must be kept below about 0.006 corresponding to 0.04 oxygen in the melt.

This can be done by controlling the water vapor-hydrogen ratio. The particular ratio, of course, is dependent upon both temperature and concentrations of the various constituents. Where the metal is a steel at 1600 C. containing 18% chromium, then a volume ratio of water to hydrogen in the innocuous atmosphere of about 0.022 to 0.032 would just protect the chromium. For practical This table rather dramatically illustrates the cifects of controlling the various variables. As can be seen, in all cases, at any given CO content in the innocuous gas the carbon content of the metal is controlled by its dissolved oxygen content which in turn is controlled by the Hgo/Hg ratio. However, to obtain low carbon content while limiting the dissolved oxygen level so as to avoid oxidizing chromium, CO concentrations must be stripped to a low concentration. For example, as shown in the table an atmosphere containing 10% CO would preclude lowering the carbon content below about 0.10% by Weight without some chromium oxidation occurring. If the CO concen. tration is kept below this level, for example at 1% as shown in the table, then the carbon content can be lowered below about 0.10% without any significant chromium oxidation occurring.

Some of the advantages of an essentially H atmosphere are that CD can be expelled without wasting argon or having the problem of stripping CO from H,, H O vapor above 0 C. can be added to supply 0 for CO, and reactions are faster, due to more gas molecules able to react being present.

The principles of the instant invention are equally applicable to the removal of carbon to produce magnetically soft silicon steels with silicon contents of from about 0.5% to about 4.5% as shown in the following examples at 1600" C.

1% silicon steel AISI Grade M-43 At this level the various activity coeflicients can be considered as unity 1.% Si is in equilibrium with SiO when it contains 0.0055 O pH O/pI-i =3.65 X O=3.65 X 0.0055=0.02

In other words, if pH O/pH is below 0.02, carbon can be oxidized to form CO without oxidizing silicon to form SiO At 0.01% maximum C,

That is, 2.75% CO by volume in the atmosphere. In other words, if pH O/pH =0.02, and C0 in the atmosphere is less than 2.75% by volume, carbon can be removed from the metal to even lower than 0.01%

5% silicon steel With this high silicon content, the same equations are applicable but the appropriate activity coefficients must be considered. 5% Si is in equilibrium with SiO when it contains 0.0064% P 2 P z= (fo In other words, if pH O/pH is below 0.0047 carbon can be oxidized to form CO without oxidizing silicon to form SiO;

%0Xf (f :0.002 poo At 0.01% maximum C,

That is 2.1% CO by volume in the atmosphere when the total pressure in the chamber is one atmosphere. In other words, if pH O/pH -=0.0047 and CO in the atmosphere is less than 2.1% by volume, carbon can be removed from the metal to even lower than 0.01%.

These two examples demonstrate that when the metal contains up to about 5% by weight silicon, the carbon content of the metal can be lowered if the water vapor content in the innocuous atmosphere is maintained at a volume ratio to the hydrogen of from about 0.004 to about 0.08 so as to maintain an oxygen concentration in the molten metal of not more than about 0.03% by weight.

It is believed that these data show that the process according to the instant invention can afford means for removing carbon to low levels without significantly oxidizing chromium, silicon, or other easily oxidizable constituents of the melt. Similarly, the process is capable of producing an essentially slag-free stainless metal or silicon-iron magnetic alloy with very low carbon contents. The data show that the process is capable of reducing scarfing costs caused by slag inclusions (formation of slag inclusions can be substantially avoided). Hence, the instant process should make possible appreciable savings in the overall cost per ton of metal.

While there has been described herein above the presently preferred embodiments of the instant process, it is to be understood that the invention is not limited thereto and that various changes, alterations and modifications can be made thereto without departing from the spirit and scope thereof as defined in the appended claims. For example, this process can be used in Conjunction with other processes for further lowering of the oxygen content of the molten metal or for performing various other treatments upon the molten metal, for example that process disclosed in copending application S.N. 580,248 filed Sept. 19, 1966.

What is claimed is:

l. A process for removing carbon from ferrous metals containing an easily oxidizable element other than carbon which comprises:

(a) providing a body of molten metal in a chamber maintained above the melting point of the metal;

(b) maintaining substantially atmospheric pressure within said chamber while passing inocuous gas thereinto in a sweeping manner over the molten metal so as to maintain an atmosphere in said chamber substantially free of undesired constituents;

(c) maintaining a hydrogen vapor content in the innocuous atmosphere above about 2% by volume and a watr vapor content in the innocuous atmosphere at a volume ratio to the hydrogen of from about 0.01 to about 0.06 so as to maintain an oxygen concentration in the molten metal of from about 0.002% to about 0.12% by weight, which reacts with carbon in the molten metal to form carbon monoxide gas with out substantially oxidizing the oxidizable elements;

(d) stripping the carbon monoxide gas from the innocuous atmosphere so as to maintain a carbon monoxide level of from about 1% to not more than 10% by volume in the atmosphere whereby a metal containing easily oxidizable elements substantially free of undesired carbon impurity is produced;

(e) and When the carbon content of the molten metal is reduced to a level below about 0.1% or less, lowering the hydrogen and water vapor content of the innocuous atmosphere so as to remove hydrogen gas from the molten metal until the hydrogen content of the metal is at a desired level.

2. The process of claim 1 wherein the easily oxidizable element is an element selected from the group consisting of, chromium, silicon, manganese, nickel and vanadium.

3. The process of claim 1 wherein the inocuous atmosphere is substantially 100% hydrogen.

4. The process of claim 3 wherein when the carbon content of the molten metal is reduced to the desired level, the hydrogen and water vapor content of the innocuous atmosphere are lowered so as to remove hydrogen gas from the molten metal until the hydrogen content of the molten metal is not more than about 6.5 parts per million.

5. The process of claim 1 wherein the chamber temperature is of from about 1600 C. to about 1750 C.

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HENRY W. TARRING II, Primary Examiner US. Cl. X.R. 

